Light emitting device with phosphor wavelength conversion

1. Field of the Invention

The invention relates to light emitting devices with phosphor wavelength conversion. More specifically, the present embodiments are directed to light emitting diode based lighting systems with phosphor, photo-luminescent, wavelength conversion to generate a desired color and/or color temperature of light.

2. Description of the Related Art

The development of solid state semiconductor devices, in particular light emitting diodes (LEDs), has opened up the possibility of a new generation of energy efficient lighting systems. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs (“white LEDs”).

As taught for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED chip and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorb a percentage of the blue light and re-emit yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor (that is the portion transmitted by the phosphor) is combined with the light emitted by the phosphor and provides light which appears to the human eye as being nearly white in color. Phosphor layers are typically placed, for example, adjacent or in close proximity to a light emitting surface of the LED die/chip from which they derive their excitation energy. Often, the phosphor layer is coated directly onto the LED die to achieve the desired intensity and color of illumination product.

As is known, the correlated color temperature (CCT) of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. The CCT of a white LED is generally determined by the phosphor composition and the quantity of phosphor incorporated in the LED.

Today, most lighting fixture designs utilizing LEDs comprise systems in which an LED (more typically an LED array to achieve a desired intensity of generated light) replaces conventional system components such as incandescent bulbs, mercury vapor and compact fluorescent lamps. In the case of lighting systems intended to generate a white light illumination product, the LEDs can comprise an array of red, green and blue direct light generating LEDs (that is LEDs without the inclusion of a wavelength converting phosphor material) or more recently an array of white LEDs.

As disclosed in U.S. Pat. No. 6,350,041, WO 2004/100226 and our co-pending U.S. patent application Ser. Nos. 11/827,890 and 11/714,464, the phosphor material can be provided remotely from its associated excitation source.

U.S. Pat. No. 6,350,041 teaches a solid state lamp based on an LED which emits radially dispersed light for room illumination and other applications. The lamp comprises a solid state light source such as an LED or laser which transmits light through a separator to a disperser that disperses the light in a desired pattern and/or changes the color of light. In one embodiment, the light source comprises a blue light emitting LED, the separator is a light pipe or fiber optic device and the disperser disperses the light radially and converts some of the blue light to yellow to produce a white light illumination product. The separator spaces the LED a sufficient distance from the disperser such that heat from the LED will not transfer to the disperser when the LED is operated at elevated currents as is necessary for room illumination. Such a lamp can disperse light in many patterns, but is particularly applicable to a radial dispersion of white light for room illumination enabling the lamp to be used as an alternative light source in existing luminaires and lighting fixtures.

Our co-pending U.S. patent application Ser. No. 11/827,890 filed Jul. 13, 2007, teaches LED lighting/illumination systems/fixtures or luminaires in which the phosphor material is located remotely from its associated excitation source (LED). Light generated by the LED is transported to the phosphor material via a waveguiding medium and provides excitation radiation to the phosphor(s) and/or phosphor layer(s), causing a photo luminescence. The light from the phosphor(s) may comprise the final illumination product, or it may be combined with any visible light from the excitation source (which may occur, for example, in the case of a blue LED as the excitation source). The final illumination product may be white light, or any colored light. The waveguide can be configured to be in the shape of a lighting system such as a hanging lighting fixture, a desk lighting fixture, a floor standing lighting fixture, a desk lamp, track lighting, spot lighting, accent lighting or a lighting panel for incorporation into a suspended ceiling. In the lighting panel configuration, the waveguide is substantially planar in form and has the phosphor provided over the entire light emitting face of the waveguide. Excitation radiation generated by a plurality of LEDs is coupled into one or more edges of the waveguide such as to propagate substantially throughout the waveguide and is then emitted through the light emitting face where at least a part of the excitation radiation excites the phosphor which emits light of a second wavelength. To prevent light being lost through the rear face of the panel a reflecting layer is provided on the rear face of the waveguide, that is the face opposite the light emitting face.

U.S. patent application Ser. No. 11/714,464 filed May 3, 2007, teaches lighting fixtures and systems in which the phosphor is provided on a shade which is located remotely from the excitation source (LED). The shade (optical enclosure) is configured to at least in part surround the LED such that excitation radiation travels, propagates in free-space (that is, it is not guided by an optical medium), a distance of at least one centimeter from the excitation source to the shade. The phosphor can be provided on the inner surface or outer surface of the shade or incorporated within the shade material. In one embodiment the lighting system comprises a bulkhead light which comprises a housing containing one or more LEDs and the shade comprises a front window covering the housing opening. The housing which is made of an opaque material includes reflective inner surfaces for reflecting light towards the shade.

WO 2004/100226 discloses an LED panel lamp comprising an enclosure housing a plurality of UV emitting LEDs and a removable front light emitting panel containing one or more UV excitable phosphor materials. Since a phosphor material generates light that is radiated equally in all directions (isotropic), some of the phosphor generated light will be directed backwards into the enclosure. To prevent such light entering the enclosure and being lost, the front panel includes a coating on its interior surface which is transmissive to the wavelengths of the LED yet reflective to wavelengths produced by the phosphor(s). Moreover, to prevent bleed through of non-converted UV light, the front panel can include a second coating on its exterior surface which is reflective to UV light and transmissive to the light generated by the phosphor(s). The document also describes a lighting system which resembles a standard incandescent light bulb in which one or more phosphor materials are included on the inner surface of the bulb envelope.

While such lighting system designs may have demonstrated properties that are acceptable to some customers—properties such as intensity, color temperature and color perception, for instance—a need exists for a lighting system based on solid-state components of even higher efficiency.

The present invention arose in an endeavor to provide a light emitting device with phosphor wavelength conversion which, at least in part, overcomes the limitations of the known devices. Embodiments of the present invention are directed to light emitting devices comprising an excitation source, which may for example be an LED or a laser diode, and a light emitting surface that includes one or more phosphor, photo luminescent, materials. Light from the excitation source provides an excitation radiation to the phosphor(s) causing a photo luminescence. The light generated by the phosphor combined with light from the excitation source comprises the illumination product of the device. In contrast to the known devices the light emitting area further comprises one or more areas (window(s)) which do not include phosphor material and which are substantially transparent to both light generated by the phosphor and by the excitation source. Such windows improve the efficiency of the device by maximizing light emitted by the device.

According to the invention a light emitting device comprises: at least one excitation source, such as one or more blue light emitting diodes, which is/are operable to generate excitation light of a first wavelength range and a light emitting surface having at least one phosphor material which absorbs at least a part of the excitation light and emits light of a second wavelength range, wherein light emitted by the device comprises combined light of the first and second wavelength ranges emitted by the light emitting surface, characterized in that the light emitting surface has at least one window area which does not include the at least one phosphor material, said window area being substantially transparent to light of the first and second wavelengths.

The light emitting surface preferably comprises a transparent substrate, such as an acrylic, polycarbonate, polythene or glass material, which is substantially transparent to light of the first and second wavelengths and which has the at least one phosphor material on a surface thereof. The phosphor material can comprise an array (pattern) of phosphor regions (islands) in which the gaps between phosphor regions defines the at least one window area. To ensure a uniform color and/or color temperature of emitted light the pattern of phosphor regions is distributed, preferably substantially uniformly, over substantially the entire area of the light emitting surface. In one such an arrangement, in which the array of phosphor regions comprises a regular array of square phosphor regions, the window area will be grid-like in form and is distributed over the light emitting surface. The phosphor regions can, for example, be substantially polygonal, square, circular, elliptical, triangular or rectangular in shape. Conversely, the phosphor material can cover the entire light emitting surface and include an array of window areas. To ensure a uniform color and/or color temperature of emitted light the pattern of window areas regions is distributed, preferably substantially uniformly, over substantially the entire area of the light emitting surface. As with the phosphor regions the window areas can, for example, be substantially polygonal, square, circular, elliptical, triangular or rectangular in shape. The array of phosphor regions and/or window areas can be a regular or irregular pattern.

The substrate can be configured as an optical component through which the excitation light and phosphor generated light passes. Alternatively, the substrate can be configured as a waveguiding (light-guiding) medium and the excitation source configured to couple excitation light into the substrate. In one such arrangement the substrate is substantially planar in form and the excitation light is coupled into at least a part of one edge of the substrate. Preferably, the device further comprises a reflector on at least a part of the surface of the substrate opposite to the light emitting surface to help ensure that substantially all light is emitted from the light emitting surface. To promote the emission of light from the light emitting surface, the light emitting surface of the substrate can include a surface topology, such as a surface patterning. The surface topology can be defined by laser or mechanical scribing the substrate surface, molding the substrate to include the surface topology or roughening the surface. The substrate can comprise other waveguide forms such as being elongate or cylindrical in form, with the light emitting surface being a substantially flat face or curved surface of the waveguide.

Where it is required to generate light of a particular color and/or CCT, the light emitting surface preferably comprises a pattern of at least two different phosphor materials. The phosphor compositions, density of phosphor material and the relative total areas of the phosphor materials and window area(s) can be used to control the color and/or CCT of the emitted light.

For a light emitting device which is intended to generate white light with a high CRI (color rendering index), the device can further comprise one or more LEDs which is/are operable to generate light of a third wavelength range which contributes to light emitted by the device by the light emitting surface. In one arrangement, blue LEDs are used to excite a green emitting phosphor material and orange or red LEDs used to generate orange or red light components of the final emission product. It will be appreciated that in such an arrangement the emission product comprises red (R), green (G) and blue (B) color components. Preferably, the ratio of blue LEDs to orange or red LED chips is substantially two-to-one to ensure that the green light contribution is sufficient to achieve a required CRI. It will be appreciated that in such an arrangement light generated by the orange or red LEDs does not result in excitation of the phosphor and such light is emitted through the one or more window areas in the light emitting phosphor surface. In an alternative arrangement blue LEDs can be used to excite an orange or red emitting phosphor and one or more green emitting LED chips used to contribute green light to the emission product.

The phosphor material can comprise: a silicate-based phosphor; an aluminate-based phosphor; a nitride-based phosphor material; a sulfate-based phosphor material; an oxy-nitride-based phosphor; an oxy-sulfate-based phosphor; a garnet material; a silicate-based phosphor of a general composition A₃Si(OD)₅ in which Si is silicon, O is oxygen, A comprises strontium (Sr), barium (Ba), magnesium (Mg) or calcium (Ca) and D comprises chlorine (Cl), fluorine (F), nitrogen (N) or sulfur (S); a silicate-based phosphor of a general composition A₂Si(OD)₄ in which A comprises Sr, Ba, Mg or Ca and D comprises Cl, F, N or S; or an aluminate-based phosphor of formula M_1−xEu_xAl_yO_{[1+3 y/2]} in which M is at least one of a divalent metal comprising Ba, Sr, Ca, Mg, Mn, Zn, Cu, Cd, Sm or thulium (Tm).

According to a further aspect of the invention a light emitting device comprises: an enclosure having a reflecting inner surface; at least one excitation source housed within the enclosure and operable to generate excitation light of a first wavelength range; and a light emitting surface covering the opening of the enclosure and having at least one phosphor material which absorbs at least a part of the excitation light and emits light of a second wavelength range, wherein light emitted by the device comprises combined light of the first and second wavelength ranges emitted by the light emitting surface, characterized in that the light emitting surface has at least one area which does not include the at least one phosphor material, said area being substantially transparent to light of the first and second wavelengths thereby allowing a part of excitation light and reflected phosphor generated light to pass through the light emitting surface.